500K resisitor in parallel with 1nf polypropelene cap - 1K resistor at the grid input of 7199 solved the problem..quite a good sound, but bit slow sounding and blur bass..wonderful vocal & high through
This approach using a LM339 to provide bias is appealing in it's simplicity. But I wonder about putting a voltage regulator directly in the signal path. How does it sound?
Jim W.
I don't know how it sounds at this point. I'm in the process of doing a total rebuild on an ST-35, which is now 75% complete, so I should be able to present my opinion in a few weeks. Dave Gillespie states that the measured impedance of the regulator is very low, so I'm guessing that the effect of having a SSD in a cathode circuit should not have any detrimental effect on sound quality. Others have been routinely using diodes, LEDs, BJTs and MOSFETs in cathode circuits without complaint, so I'm confident of good things to come.
Last edited:
The voltage regulator takes care of DC and very low frequencies. The bypass capacitor handles higher frequencies i.e. most audio. The net result should be better than an unmodified circuit, even if the regulator output impedance is a bit non-linear. A rather clever upgrade! Like all the best ideas it is simple.
My concern was with the regulator's dynamic response, especially at high frequencies. But now that you point it out (and I open my eyes while looking at the schematic), C24 provides a signal path to ground so that's not so much of a concern. Clever mod.
Jim W.
I just got one sca35. It seems to have a problem wity the preamp section. According to the schematic 1st half of 12ax7 is for phono, second half is for high level input eg radio, tape etc.
The schematic state that b+ for the 2nd half of 12ax7 should be 180v. In my amp it's only about 130v on both channel. Gain seem lacking when i listen to it i have to turn up volume pass 12 oclock.
Anyone has suggest where the problem is? B+ on the firzt half of 12ax7 seems correct according to schematic 105 v. I under stand that phono amp usually run at lower b+
The schematic state that b+ for the 2nd half of 12ax7 should be 180v. In my amp it's only about 130v on both channel. Gain seem lacking when i listen to it i have to turn up volume pass 12 oclock.
Anyone has suggest where the problem is? B+ on the firzt half of 12ax7 seems correct according to schematic 105 v. I under stand that phono amp usually run at lower b+
Quan -- On an SCA35, the 12AX7 tubes only serve as a preamplifier for phono or tape head service. When hi level inputs are selected, these tubes are not used at all; the input is sent straight to the passive control section, and straight into the power amp boards. While the gain for high level inputs is usually adequate, it is quite normal for the volume control to operate at a straight up position or higher based on the source's signal level. The input sensitivity on the hi level inputs for the SCA35 is 1.0 vac for full power output.
Dave
Dave
Thanks Dave, i looked at the schematic again and it seems that the spare input goes directly into the passive control section. But the radio input and "from tape amp" input seems to go through the 12ax7. I might be confused by the drawing of the selector switch.
I have a turntable and also need to use the phono input. Still wondering why b+ on 2nd half of 12ax7 is much lower than spec in the manual and it's the same on both channel.
Cheers
I have a turntable and also need to use the phono input. Still wondering why b+ on 2nd half of 12ax7 is much lower than spec in the manual and it's the same on both channel.
Cheers
Quan -- I think you are being confused by the drawing of the selector switch. As drawn, it is shown in the "Tape Head" position, the most CCW position. If you rotate the connector ring clockwise, you will see that the last three positions connect the three high level inputs directly into the passive control circuits.
Besides off value plate load or cathode resistors, the only thing that could really cause the plate voltage to drop on the second section of the 12AX7s (assuming "normal" 12AX7s are installed), is leaking coupling caps between the first and second sections of these tubes. Check to make sure that the voltage on pin #2 is no more than 0.00 vdc on these tubes. It should in fact even be negative, which is normal for the way these stages are operated.
Dave
Besides off value plate load or cathode resistors, the only thing that could really cause the plate voltage to drop on the second section of the 12AX7s (assuming "normal" 12AX7s are installed), is leaking coupling caps between the first and second sections of these tubes. Check to make sure that the voltage on pin #2 is no more than 0.00 vdc on these tubes. It should in fact even be negative, which is normal for the way these stages are operated.
Dave
Hey All,
I'm rebuilding an SCA-35 right now. I was planning to get rid of the 7199 and replace it with a 6AN8A by rewiring the socket. But after some research this may not be a good idea? Then I looked at the ST-35 schematic. It seems very similar. Any reason why I couldn't use the ST-35 values on the SCA board? Then I could use the 12DW7/7247 twin triode instead.
Kevin
I'm rebuilding an SCA-35 right now. I was planning to get rid of the 7199 and replace it with a 6AN8A by rewiring the socket. But after some research this may not be a good idea? Then I looked at the ST-35 schematic. It seems very similar. Any reason why I couldn't use the ST-35 values on the SCA board? Then I could use the 12DW7/7247 twin triode instead.
Kevin
Last edited:
Kevin -- The main difference between the SCA-35 board and the ST-35 board is that the SCA board has significantly more gain to it (hence the use of the 7199) to account for the loss through the tone control networks. If you are bypassing those networks and only using a level control, then use of the ST board will produce a very similar gain characteristic to that of the SCA board with the tone controls left in place. Alternately, the 6AN8 can replace the 7199 in terms of performance, but would require not only a rewiring of the socket, but also an adjustment to the screen dropping resistor, and an adjustment to the HF stability circuits as well.
Dave
Dave
"I'd skip the 7247 and use 12AX7 and 12AT7. You can build it on a perf board."
An amp builder designer on another form suggested a 6AQ8/6CG7 as he is one of a number that doesn't like the 12AX7.
I have a used pair of Z-565 OPT's and a used 272HX (300-0-300V, 200 mA) and so interested too.
Would these tubes just plug in with socket rewiring? Both those tubes are in production so I like that too.
Thanks in advance!
Regards,
Randy
An amp builder designer on another form suggested a 6AQ8/6CG7 as he is one of a number that doesn't like the 12AX7.
I have a used pair of Z-565 OPT's and a used 272HX (300-0-300V, 200 mA) and so interested too.
Would these tubes just plug in with socket rewiring? Both those tubes are in production so I like that too.
Thanks in advance!
Regards,
Randy
Thanks Sy and Dave,
This amp won't be a powerhouse regardless. The purpose of this exercise is to build a nice little amp that will go to someone that hasn't had a tube amp before. Without completely re engineering the amp.
With that in mind I can use the 12DW7's with the ST35 values. Remove the 'silly switches' Stereo/Mono, Filter, and the Loudness Contour. Dave's EFB feedback design and let the gain fall where it will. Then make it pretty with a new faceplate and sell it to BUY MORE TUBE PARTS!
Thanks again Guys
This amp won't be a powerhouse regardless. The purpose of this exercise is to build a nice little amp that will go to someone that hasn't had a tube amp before. Without completely re engineering the amp.
With that in mind I can use the 12DW7's with the ST35 values. Remove the 'silly switches' Stereo/Mono, Filter, and the Loudness Contour. Dave's EFB feedback design and let the gain fall where it will. Then make it pretty with a new faceplate and sell it to BUY MORE TUBE PARTS!
Thanks again Guys
mr2racer:
You've about described the SCA mod I've built, I used stereo 35 repro boards,
added EFB mod, used bigger power supply filter caps, and 3 rc sections between the power amp and driver sections. It sounds good! With the EFB mod, it can go loud cleanly even with moderate effeciency speakers. Typically I've got the volume knob at 8 to 11 o'clock with CD source, so the gain is fine.
I have lowered the feedback a little from stock, but before that, I still
wasn't higher that 12 o'clock on the volume in normal listening.
You've about described the SCA mod I've built, I used stereo 35 repro boards,
added EFB mod, used bigger power supply filter caps, and 3 rc sections between the power amp and driver sections. It sounds good! With the EFB mod, it can go loud cleanly even with moderate effeciency speakers. Typically I've got the volume knob at 8 to 11 o'clock with CD source, so the gain is fine.
I have lowered the feedback a little from stock, but before that, I still
wasn't higher that 12 o'clock on the volume in normal listening.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.